With rising energy costs, especially high electric costs, and electricity use restrictions placed on heat treating companies in many states and countries, the need to develop more energy efficient heat treating furnace hot zones is a key priority. The furnace hot zone is the area within the furnace chamber in which a work piece is placed to be heat treated and consists of a furnace hearth for placing a workload to be heat treated, heating elements, a power supply terminal, cooling gas nozzles, and an insulation package to ensure that the temperature is uniform throughout the heating cycle. The present invention includes some notable improvements over prior art hot zone arrangements for saving energy and reducing the overall costs of manufacturing, owning and operating a vacuum furnace. A uniquely designed prefabricated insulation assembly, heating elements and their connection joints, and lower mass cooling nozzles, result in improved energy consumption by the vacuum furnace, easier furnace fabrication and maintenance, and a significant reduction in the initial cost to build the furnace compared to current graphite vacuum furnace hot zones.
Less mass in the furnace hot zone reduces the time for both heating and cooling the furnace and the workload placed in the furnace to be heat treated. A reduced mass hot zone in a high temperature and high pressure gas-cooled vacuum furnace must be able to withstand the force of the cooling gasses at the high pressures needed to achieve the desired metallurgical properties for various tool steels, alloy steels and aerospace alloys that are being processed. The hot zone requires some form of support so that its components remain stable during both the heating and cooling cycles. The key support feature of all current and older vacuum furnaces, featuring gas cooling pressures above atmospheric pressure, utilize a continuous, fully enclosed steel support ring enclosure. This ring enclosure connects and retains all the components of the hot zone, insulation package, heating elements, hearth, gas nozzles and power terminals.
It is well known in prior art vacuum furnace fabrication that the hot zone contains an inner insulating wall and a fully enclosed outer wall known as the support ring—U.S. Pat. Nos. 9,187,799; 7,514,035; 4,559,631; 4,259,538; 4,489,920; 6,021,155; US2013/0175256A; and U.S. patent application Ser. No. 15/330,396. The outer wall support ring typically is fabricated as a stainless steel or carbon steel ring and is situated and isolated within a water-cooled chamber. The inner insulating wall typically is fabricated with all metal radiation shields or a combination of graphite felt and foil, or rigidized graphite board. In one instance described in U.S. Pat. No. 4,489,920, and in U.S. patent application Ser. No. 15/330,396, a standard, fully enclosed stainless steel support ring is utilized to support the insulation ring package in the vacuum furnace hot zone. The present invention is an improvement over the prior art in that it eliminates this high mass stainless steel support ring and further reduces energy costs by reducing the overall mass of the furnace hot zone. The present design, as described and claimed in this application, yields a substantial reduction in energy costs over prior art vacuum furnace designs that utilize a stainless steel support ring in the hot zone.
A key feature of the present ringless support design is the glued HEFVAC boards at the juncture of their respective tongue-and-groove edges, using a high strength, high temperature graphite glue or sealant to form a single polygonal structure that does not require substantial support, as do prior art insulation ring packages. The insulation board ring assembly is constructed from a plurality of at least 3 inch thick HEFVAC high density, high strength and low moisture-sensitive graphite insulation boards designed with a tongue-and-groove longitudinal edge, and glued to each adjacent board at each respective connective tongue-and-groove juncture. The glued boards are cured in a two step process. First, air is immediately circulated on the glued tongue-and-groove juncture, and then the glued insulation board ring assembly is cured in a high temperature vacuum heat treatment cycle to produce a single unit insulation assembly ready for installation and placement on the support structure in the furnace hot zone by the furnace manufacturer. The use of the glued insulation board ring assembly reduces radiative losses in the furnace hot zone, since the fluid nature of the glue fills any gaps that were inherent in prior art designs. The shape of the cured insulation board ring assembly is preferably polygonal and can be supported from the bottom only on the support structure, which contacts and surrounds the bottom approximately 35-40% portion of the polygon-shaped insulation board ring assembly. The use of a partial support base instead of the prior art full stainless steel support ring decreases the overall mass of the support structure by approximately 80-85%.
The use of a fully prefabricated and glued HEFVAC graphite insulation board assembly in the hot zone, as described and claimed in the present application, further reduces labor costs to construct the insulation assembly in the hot zone by the furnace manufacturer as in prior art designs, providing substantial savings. The cost savings for manufacture and for heating and cooling operations in the furnace are key features of the present invention over the prior art.
In a preferred embodiment of the present invention the support structure consists of a series of brackets placed strategically throughout the bottom and edges of the chamber wall to support the ringless hot zone.